Respond with a 404 when the file doesn't exist

Prior to this, I was seeing the successful HTTP status 200 when
requesting files that did not exist.

With this change, the server sends back Nginx's default 404 page instead
of the home page.

conf/app-nginx.conf.sigil

@@ -23,7 +23,7 @@ http {
     port_in_redirect off;
 
     location / {
-      try_files $uri $uri/ /index.html;
+      try_files $uri $uri/ =404;
     }
   }
 }